The present application has been filed concurrently with U.S. Pat. applications, Ser. No. 859,609, filed May 5, 1986 and Ser. No. 859,611, filed May 5, 1986, and assigned to the assignee of the present application.
1. Field of the Invention
The present invention relates to the field of mechanical shakers and more specifically to electromagnetically driven armature shakers.
2. Prior Art
There are a considerable number of shakers that are well-known in the prior art. These shakers are used to mechanically shake an item for the purpose of diagnostically testing responses to certain driving forces. The item is physically attached to a moving portion of a shaker and when the shaker is activated, the item is subjected to a variety of test conditions. The moving portion of the shaker is typically driven by a force which may be continuous, cyclical or impulsed.
One class of these shakers employs the use of an electrodynamically driven armature to provide the shaker's mechanical movement. Typically, a housing unit contains a stationary field coil which is connected to an external power source. A freely moving armature located within the housing contains an armature coil which is driven by a power amplifier. The armature coil is fixed to the shakaing portion of the unit, such as an armature frame.
By providing a varying drive signal from the amplifier to the armature coil, the armature will respond and move accordingly due to a change in the electromagnetic field. The size and direction of the shaker force is determined by the strength of the driving signal from the power amplifier and the power applied to the stationary field coil.
A variety of platforms or tables are normally coupled to the armature, wherein the item to be subjected to testing is mounted onto the table by fixed means, such as bolts, screws, etc. The platform or table is usually constructed from a hard substance, such as metal, to prevent deformation of the platform which will interfere with the resultant testing.
One side-effect from using such an armature frame is the generation of unwanted frequency components from the mechanical movement of the shaker armature. These unwanted frequencies are generated as a result of oscillations created within the moving metal. Such oscillations finally reach a peak resonant frequency of the particular material in motion and tend to have disruptive effects by producing undesirable additional factors during the testing cycle. A number of types of resonance are encountered in prior art armature frames. One is the telescoping resonance associated with the height of the table and the other is the bending resonance associated with the mass/stiffness distribution of the table.
Another problem encountered in prior art shakers is the stress placed on central armature shaft bearings. Typically, an armature is placed in the shaker housing, such that the armature coil windings fit within the circumference of the field windings. A shaft attached to the armature frame couples the armature to the housing, but, because the armature requires movement, a surface having a low coefficient of friction is necessary.
To mechanically couple the armature shaft to the shaker housing, a bearing is normally used. Metal roller or ball bearings provide a lower coefficient of friction yet have the strength to support a substantial load. Further, because most armatures only have linear movement (up and down motion), prior art linear motion bearings have been used. One such linear motion ball bearing is disclosed in U.S. Pat. No. 3,900,233, wherein several linear ball bearing tracks have been designed in the interior wall of the bearing housing. However, such a track design limits the loading factor on the ball bearings, because each track must be spaced so as to allow for the placement of the non-loading return track.
A further problem is associated with means to stabilize the shaker armature during operation. Typically, an armature shaft is smaller in diameter in comparison to the diameter of the armature. To prevent table wobble, some retainer means are needed to support the armature. Prior art methods have used springs and rubber bushings, if any support were used at all.
What is needed then is an improved shaker assembly having an armature which provides a test result which is more independent of the resonant frequencies encountered in the equipment, a bearing which allows a better load transfer at a reduced coefficient of friction, and a supporting mechanism which produces high lateral load capability and lateral stiffness with little vibration interference, if any.